1. Field of the Invention
The present invention relates to methods for forming holes in reinforced metal matrix composite materials, and more particularly to an economical method for drilling holes through reinforced metal matrix composite materials without drill breakage or loss of dimensional accuracy.
2. Background of the Invention
Today, materials used for advanced design aerospace structures must exhibit exceptional structural strength characteristics, while at the same time permiting significant reduction in overall weight. One class of such materials is reinforced metal matrix composites.
Metal matrix composites are generally comprised of a hard, high-strength reinforcement material distributed within and throughout a softer, more ductile metal or alloy. The reinforced material may be a hard, abrasive ceramic material, such as silicon carbide or aluminum oxide, and may be in the form of powder or particles, short fibers or whiskers, or long continous fibers. The matrix material may be aluminum, copper, titanium or some other metal or alloy.
There are a variety of methods in use today for fabricating metal matrix composites, including casting and infiltration, vapor deposition, solid state diffusion at high temperatures and pressures, etc. The resultant metal matrix composite products, in whatever manner produced, commonly exhibit a similar characteristic, namely the inclusion of a hard, abrasive ceramic phase in a softer metal matrix.
One problem in working with reinforced metal matrix composite materials is how to form holes or openings for receipt of fasteners, principally for the purpose of fastening members together.
One approach to forming holes in reinforced metal matrix material has been to punch through the thickness of the consolidated matrix material. However, this method has been found to be undesirable insofar as the typically soft matrix material 12 tends to deform in the direction of the punch, and the fibers 14 running to the edge of the hole 16 are similarly affected, as shown in FIG. 1 of the drawings.
Another technique used to form such holes has been to drill through the consolidated material at low feed rates and at high rpm using a diamond core drill. These core drills are extremely expensive, and generally perform well only in extremely hard materials, such as ceramics, glass and carbon reinforced epoxies. While the composite reinforcement is hard and brittle, the metal matrix material is much softer, more ductile and prone to galling, which causes excessive heating and deterioration of the diamond core drill. This leads to rapid loss of dimensional accuracy and a resultant, very high, dollar-per-hole cost. In fact, diamond core drill wear occurs so rapidly that the cost per hole often exceeds $45.00.
Another method for drilling in metal matrix materials has been to use conventional drill bits made from hard, wear resistant materials, such as carbide. This method, when using conventional drilling parameters, has resulted in rapid wear or high incidence of breakage of the drill bit.
A process which facilitates the drilling of holes through reinforced metal matrix materials with minimized drill breakage and cost per hole would therefore be most desirable.